Re: RS485 is bidirectional does it mean it is fullduplex?

Grant Edwards <grante@xxxxxxxx> wrote:
>On 2005-06-16, Floyd L. Davidson <floyd@xxxxxxxxxx> wrote:
>> Grant Edwards <grante@xxxxxxxx> wrote:
>>>On 2005-06-15, Floyd L. Davidson <floyd@xxxxxxxxxx> wrote:
>>>
>>>> RS-485 is a differential (balanced) system, and there is no
>>>> signal ground connection. The cable used might well include a
>>>> frame ground, but that is for noise induction cancellation, not
>>>> signal ground.
>>>
>>>In my experience, the third/fifth wire is required to limit the
>>>common mode voltage seen by the receivers. In that respect, it
>>>is a signal ground. IIRC, most receivers them can only tolerate
>>>8-12V common-mode DC. If you let the two devices float with
>>>respect to each other, you can get fairly high common-mode
>>>voltages and the recievers will stop working.
>>
>> That is a frame ground, and not a signal ground. It will carry
>> no signal current at all.
>
>It doesn't carry any signal current, but it is the ground to
>which the receiver's input signal range specs are references.
>It's the ground that defines what "0V" is for the signal
>inputs. I call that the signal ground.

It's a frame ground. A signal ground would be a return path to
ground for the signal. RS-232 has such a signal, and is "single
ended", hence all signal lines share the same common ground
return line.

Differential ciruits are "balanced", the signal is between the
two wires of a pair. Neither of them is at ground, so neither
is called a "signal ground". The signal does not depend on
any relationship to ground.

If course, the receiver typically cannot tolerate a common mode
voltage greater than some specified voltage. That that is not
a signal voltage in any way. It just biases the devices out of
their useful dynamic range.

Optical isolators are are nice because they have a significantly
higher "useful dynamic range". Otherwise, the signal on the
cable is still the same.

>> And any variation of current seen will be strictly noise.
>
>What current?

Induced current into that ground wire accompanying the signal
pairs. That's why you don't want it to be one small wire if
it in the same bundle as the signal pairs.

>> The trick is to get the induction into the ground wire to
>> then, in the cable between the ground wire and the signal
>> pairs, cancel the induction into the signal cables.
>
>I really don't understand what you're talking about. The

It is a bit complex. Many people who work with cables don't
understand it very well. But any one who sits down and studies
it a bit can understand it. It isn't even rocket science! ;-)

>differential receiver inputs can deal with only a few volts of
>common mode DC voltage. You have to use a ground that's common
>between the transmitters and receivers to make sure that the
>common-mode DC voltage seen by the receivers is within spec.

Yes. You also have to be very careful about the currents induced
into said ground connection. Do it the wrong way, and it adds
noise to the signal pairs; do the right way and it will help
cancel noise induced from the same source into those signal pairs.

>> What kind of distances have you tried that with?
>
>A couple kilometers.

That's a pretty good run for RS-485.

>> I'd expect that across the room or around the bend might be
>> just fine (and wouldn't be needed because the offset between
>> the ground systems wouldn't be high enough to be a problem).
>> But if this went down the road 3000-4000 feet, and you
>> actually did get a ground offset high enough to be a problem,
>> using a single wire in the same cable to equalize the ground
>> potential should add enough noise to your cable run to make it
>> a real problem.
>
>It didn't seem to.

What kind of cable was this? Cable you installed, or telco
cable?

>> A proper ground on each would be much better.
>
>Not allowed for safety reasons. The RS-485 transceivers at
>both ends are optically isolated from earth.

The cable still has to be grounded at both ends. That connects
your two frame grounds together too.

>> And a cable sheath that is properly grounded at *both* ends,
>> to the same single point building ground that the RS-485
>> equipment is tied to, would be the preferred way to make sure
>> there wasn't too much common mode difference.
>
>Nope. The cable sheild is earth ground at one end or the other
>and can't be electrically connected to the RS-485 signal or
>"ground" signals.

It *should* be connected to earth ground at both ends. And that
ground point *should* be a single point where *all* frame
grounds for the entire building go.

Typically equipment bays in a single row are strapped together,
though sometimes individual racks will have separate grounds.
There should be a single cable from each row (or each rack if
some racks are isolated) to a common grounding point on each
floor of a building. Each comm cable entrance would be
considered just like an individually isolated rack, and would
have its own ground cable going to the grounding point for that
floor. (What this says is that the cable is *not* connected to
a rack. That a rack in one row is *not* connected to a rack in
a different row. That no two rows share a single cable going to
the ground point. But often racks in one row share a ground
cable, and often equipments mounted in one rack share ground
wires.)

Both ends of the cable should be grounded in that fashion.

--
Floyd L. Davidson <http://web.newsguy.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska) floyd@xxxxxxxxxx
.

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